Double wall acoustic panel

ABSTRACT

The invention relates to an acoustic panel comprising first and second walls having a sound absorber disposed therebetween, wherein the first wall presents, on an inside face facing towards the sound absorber, a plate of viscoelastic material and a backing plate, spacer elements being distributed in at least one dimension and creating an air gap between the absorber and the backing plate.

The present invention relates to a double-walled acoustic panel having asound absorber disposed between the walls.

BACKGROUND OF THE INVENTION

In such a panel, absorption efficiency can be improved by creating anair gap between the sound absorber and the wall that receives the soundexcitation.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention relates to an acoustic panel of theabove-specified type of structure that makes that it possibleadvantageously to create such an air gap.

The invention thus provides an acoustic panel comprising first andsecond walls between which a sound absorber is disposed, wherein thefirst wall presents on an inside face facing towards the sound absorber,a plate of viscoelastic material, and also a backing plate carryingspacer elements (e.g. parallel strips, honeycomb, embossing, and/orstuds) that are distributed in at least one dimension and that create anair gap between the absorber and the backing plate.

More particularly, the spacer element may form a two-dimensional array,e.g. in the form of a grid or a honeycomb or embossing, or indeed in theform of studs distributed in said two dimensions.

The grid, the honeycomb, or the embossing may constitute a net or baghaving the sound absorber placed therein. In particular, the net or bagmay be made of a thermally conductive material so as to constitute aheat sink between said walls.

A self-adhesive film carrying said array (e.g. grid, honeycomb,embossing, and/or studs) may be stuck onto the sound absorber, inparticular onto packaging of the sound absorber, or onto the backingplate.

In a preferred variant, the backing plate is stiffened by embossing, byoptionally parallel ribs, by a honeycomb, by studs, or by bosses.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear better onreading the following description given by way of non-limiting exampleand with reference to the accompanying drawings, in which:

FIG. 1 is an exploded view of an embodiment of the invention; and

FIGS. 2 to 6 show variants of the invention.

MORE DETAILED DESCRIPTION

In FIG. 1, the acoustic panel presents a first plate 1 which receivessound excitation (arrow F) when the panel is in place, a second wall 2,and a sound absorber 3, e.g. glass wool which may be contained in a finecase 4 made of “Mylar” (trademark filed in the name of DuPont).

The plate 1 presents on its inside face a plate 17 made of aviscoelastic material (e.g. the material named “Deltane” from thesupplier Paulstra).

An air gap is provided by using a mechanical spacer constituted by agrid or by embossing 5 which may be secured to a rigid backing plate 10,or indeed by a embossed backing plate.

The mesh shape may be arbitrary, however its size must be such thatunder the action of the pressure to which the absorber is subjected, theabsorber does not move beyond the area defined by the mesh, i.e. it doesnot come into contact with the backing plate 10, and the air gapmaintains the desired thickness.

The nature of the material constituting the grid is selected as afunction of environmental constraints for the intended application. Thematerial may be malleable so as to allow the grid to be fitted to shapesthat are complicated, providing that during deformation and under thepressures involved during assembly or use the absorber 3 cannot comeinto contact with the backing plate 10.

The grid 5 as defined may be stuck onto the inside face of the backingplate 10, taking care to ensure that the adhesive does not fill the meshof the grid so as to avoid any contact with the sound absorber.

The absorber 3 is of low-density glass wool contained in a fine “Mylar”(registered trademark) case. The spacer may be a grid of stainless steelwire having a diameter of 0.5 millimeters (mm) with a square mesh havinga side of 10 mm. The reduction in transparency is of the order of 3decibels (dB) to 4 dB.

The grid 5, e.g. a square grid (FIG. 2), may be replaced by a honeycomb6 (FIG. 3) of suitable size. Too small a mesh would lead to a largeconnection area between the backing plate 10 and the absorber 3, therebyreducing performance.

The grid could be replaced by studs 8 (FIG. 4) regularly distributedover the inside surface 11 of the backing plate 10. This can be done byusing a self-adhesive film 9 carrying the studs 8 and stuck to thesurface 11. Alternatively, the film 9 may equally well be stuck to theabsorber 3 or to its packaging 4, as is applicable to certain kinds ofglass wool.

Nevertheless, care should be taken to ensure that the absorber is notflattened since that would facilitate contact between the absorber andthe backing plate 10 and thus would contribute to undesired couplingwith the wall 1 that receives the excitation.

The grid 5 may constitute a net 7 in which the absorber 3 is placed.

In a variant implementation of the invention, the plate 1 presents onits inside face a plate 17 of viscoelastic material (e.g. the material“Deltane” from the supplier Paulstra), and a backing plate 10 which isembossed so as to stiffen it and which serves to damp the vibration ofthe plate 1 that is subjected to the excitation. This configurationenables the backing plate 10 to be lighter in weight (and possibly alsothe plate 17) by reducing its thickness while maintaining its stiffnessby means of walls 18 extending perpendicularly to its surface. Thesewalls act as a spacer to prevent the absorber coming into contact withthe backing plate (FIG. 9).

In the embodiments of FIGS. 5 to 7, the backing plate 10 may beconstituted by an adhesive plate 10 carrying strips 18 in one or twodimensions.

In FIG. 6, the spacers 18 are ribs of the backing plate 10 arranged in asingle dimension and spaced apart from one another, generally at aconstant pitch.

In FIG. 7, the ribs 18 (distributed in one or two dimensions) on thebacking plate 10 are wide enough to allow the plate 17 of viscoelasticmaterial to occupy the hollow portion 19 thereof (see in particular thedetail of FIG. 7).

EXAMPLE 1

A wall 1 of 1.2 mm thick aluminum was damped firstly by a sheet 17 of 1mm thick viscoelastic material and secondly by a plane 0.5 mm thickbacking plate 10 of aluminum.

EXAMPLE 2

A wall 1 of 1.2 mm thick aluminum was damped firstly by a 0.5 mm sheet17 of viscoelastic material and secondly by a 0.3 mm thick backing plate10 of aluminum embossed with a square mesh having a side of 10 mm and arib height of 2 mm (cf. FIG. 6).

For excitation of 0.1 g applied to a 1.2 mm thick plate 1 of aluminumhaving dimensions of 210 mm×230 mm, FIG. 8 shows vertical accelerationlevel in meters per second per second (m/s²) as a function of frequencyF (in the range 0 to 3000 hertz (Hz)), for the following circumstances:

curve I: said plate 1 alone;

curve II: said plate 1 damped by a 1 mm thick sheet 17 of viscoelasticmaterial (“Deltane”) and a 0.5 mm thick backing plate 10 of aluminum;and

curve III: said plate 1 damped by a 0.5 mm thick sheet 17 ofviscoelastic material (“Deltane”) covered in its center by a 0.3 mmthick backing plate 10 of aluminum (150 mm×15 mm) embossed with a squaremesh having a side of 10 mm and a rib height of 2 mm.

It can be seen that the vibration peaks (curve I) are well damped inboth circumstances (curves II and III). For comparable or betterperformance, stiffening the backing plate 10 makes it possible to reducethe thickness both of the viscoelastic sheet 17 and of the backing plate10, thereby achieving a significant saving in weight.

1. An acoustic panel comprising first and second walls between which asound absorber is disposed, wherein the first wall presents on an insideface facing towards the sound absorber, a plate of viscoelasticmaterial, and also a backing plate carrying spacer elements that aredistributed in at least one dimension and that create an air gap betweenthe absorber and the backing plate.
 2. An acoustic panel according toclaim 1, wherein the spacer elements form a two-dimensional array.
 3. Anacoustic panel according to claim 1, wherein said array comprises agrid, a honeycomb, or embossing.
 4. An acoustic panel according to claim3, wherein the grid, the honeycomb, or the embossing constitutes a netwith the sound absorber being placed inside the net.
 5. An acousticpanel according to claim 2, wherein the array comprises studsdistributed in said two dimensions.
 6. An acoustic panel according toclaim 2, including a self-adhesive film carrying said array, said filmbeing stuck onto the sound absorber, in particular onto packaging of thesound absorber.
 7. An acoustic panel according to claim 1, wherein saidspacer elements are reinforcing elements secured to the backing plate.8. An acoustic panel according to claim 7, wherein said reinforcingelements are embossing, in the form of optionally parallel ribs, ahoneycomb, studs, or bosses.